1. Field of the Invention
The present invention relates generally to methods and apparatus for controlling brushless motors used for driving compressors in automotive air conditioning system. In particular, the present invention relates to methods and apparatus for controlling brushless motors used for driving compressors in automotive air conditioning system. Such compressors are driven by electricity used in electrically driven vehicles, such as hybrid vehicles, fuel-cell vehicles, or the like.
2. Description of Related Art
Electrically driven vehicles including hybrid vehicles, fuel-cell vehicles, or the like, which have electric driving sources, have been developed in order to reduce environmental contamination. Such electrically driven vehicles generally have an air conditioning system including a compressor driven by a brushless motor. An inverter delivers driving power in three-phase current to the brushless motor.
Apparatus for controlling such brushless motors are known in the art. Such known apparatus may include an inverter. For example, known apparatus for controlling brushless motors are disclosed in Japanese Unexamined Patent Publication Nos. 2001-103785, 2001-119984, and 2001-78485. In such known apparatus, direct current (DC) is provided from a DC power source, e.g., a battery, to the inverter. The inverter controls a plurality of switching elements by switching a state on-and-off to achieve phase switching. As such, power output from the inverter is generated in the form of a three-phase-current and is provided to the brushless motor. At the same time, the switching time for each switching element in the on-state is controlled by a known pulse width modulation (PWM) control. An electric power supplied to the brushless motor may be varied, and a rotational speed of the brushless motor may be adjusted. As a result, the rotational speed of the compressor in the air conditioning system for a vehicle may be adjusted, and the temperature in a compartment of the vehicle may be adjusted.
As shown in FIG. 8, one example of a known apparatus for controlling a brushless motor using a PWM control is depicted. The known apparatus includes a switching elements group 92 having six switching elements 92U, 92V, 92W, 92X, 92Y, and 92Z, a filter circuit 94, a controller 95, a memory 96, and a voltage detector 97. Controller 95 detects a rotational position of a brushless motor 93 via filter circuit 94. Subsequently, controller 95 may drive switching elements 92U, 92V, 92W, 92X, 92Y, and 92Z based on the detected rotational position. A direct current output from a DC power source 91 may be converted to a three-phase current at switching elements group 92, and the three-phase current may then be provided to motor 93.
In addition, as shown in a timing chart of FIG. 9, switching the state of switching elements 92X, 92Y, and 92Z on and off on a lower side of the inverter is controlled by the PWM control, so that an electric power supplied to motor 93 is varied. As a result, the rotational speed of motor 93 may be adjusted.
Nevertheless, when switching elements 92U, 92V, 92W, 92X, 92Y, and 92Z are switched on and off, due to the influence of stray inductors L1 and L2 shown in FIG. 8, an upsurging voltage (hereinafter a xe2x80x9ctransient voltagexe2x80x9d) may occur between an emitter and a collector of switching elements 92U, 92V, 92W, 92X, 92Y, or 92Z when switched into the off-state because of the transient voltage phenomena. When the flow of current is increased in motor 93, a frequency with which the state of each switching elements 92U, 92V, 92W, 92X, 92Y, or 92Z is switched. As a result, the maximum value of the transient voltage may be increased. Therefore, the transient voltage may affect a threshold voltage in selecting each switching elements 92U, 92V, 92W, 92X, 92Y, or 92Z.
As shown in FIG. 10, a first transient voltage V1 occurs between an emitter and a collector of switching elements 92U, 92V, and 92W in the off state, when switching elements 92U, 92V, and 92W at an upper side of the inverter are switched on and off. A second transient voltage V2 occurs between an emitter and a collector of switching elements 92X, 92Y, and 92Z in off-state, when switching elements 92X, 92Y, and 92Z are switched on and off at the lower side of the inverter by the PWM control. Moreover, both the first transient voltage V1 and the second transient voltage V2 are greater than the voltage in a normal condition. This relationship between voltages is known in the art.
In addition, as shown in FIG. 11, if duty ratio of the PWM control is varied, the timing of the occurrence of the second transient voltage V2 is advanced, and waveforms of the first transient voltage V1 and the second transient voltage V2 overlap. As a result, the first transient voltage V1 and the second transient voltage V2 are combined, and a third transient voltage V3 may occur. The maximum value of third transient voltage V3 may be greater than that of first transient voltage V1 and second transient voltage V2. Due to the occurrence of the third transient voltage V3, certain problems may arise. First, if the third transient voltage V3 exceeds a maximum allowable voltage Vmax of switching elements 92U, 92V, 92W, 92X, 92Y, or 92Z, switching elements 92U, 92V, 92W, 92X, 92Y, or 92Z may be damaged or destroyed. Second, in order to avoid damaging switching elements 92U, 92V, 92W, 92X, 92Y, or 92Z, if the switching elements having a greater maximum allowable voltage Vmax are selected, the cost of the switching elements may increase. Third, a noise occurring at the inverter may be increased due to the presence of the third transient voltage V3. A malfunction of electric circuits in the inverter may occur due to the noise, and the noise may affect another electrical components. Fourth, in order to suppress the third transient voltage V3, a large, transient voltage absorbing circuit may be necessary. As a result, the manufacturing cost and the size of the apparatus for controlling the brushless motor may be increased.
Therefore, a need has arisen for apparatus for controlling brushless motor and methods of employing such apparatus that overcome these and other shortcomings of the related art. A technical advantage of the present invention is that negative effects due to a transient voltage, occurring at the time during which switching elements are operated or cycled, may be reduced in a low cost.
According to an embodiment of the present invention, a method or apparatus for controlling a brushless motor has an inverter providing a current to the brushless motor. The inverter comprises a first plurality of switching elements, a second plurality of switching elements, and a drive control device. The first plurality of switching elements are connected between a first terminal of a DC power source and a phase terminal of each coil of the brushless motor. The second plurality of switching elements arc connected between a second terminal of the power source and a phase terminal of each coil of the brushless motor. The drive control device selects a state of the first plurality of switching elements and switches either the state of the first plurality of switching elements or the state of the second plurality of switching elements by mean of a pulse width modulation (PWM) control, so that the drive control device controls a rotational speed of the brush less motor. The switching of the state of the first plurality of switching elements by the PWM control is altered, i.e., advanced (made to occur earlier) or delayed (made to occur later), such that a combined voltage of a first transient voltage and a second transient voltage is less than a voltage limit, e.g., a maximum allowable voltage, of the first plurality of switching elements or of the second plurality of switching elements. The first transient voltage is generated between an input terminal and an output terminal of each second switching element in the off-state, when the state of the second plurality of switching elements is switched. The second transient voltage is generated between an input terminal and an output terminal of each first switching clement in the off-state, when the first plurality of switching elements is switched. In another embodiment, the switching of the state of the second plurality of switching elements by the PWM control is delayed, such that a combined voltage of a first transient voltage and a second transient voltage is less than a voltage limit, e.g., a maximum allowable voltage, of the first plurality of switching elements or of the second plurality of switching elements. The second transient voltage is generated between an input terminal and an output terminal of each second switching element in the off-state, when the state of the first plurality of switching elements is switched. The second transient voltage is generated between an input terminal and an output terminal of each second switching element in the off-state, when the second plurality of switching element is switched.
Other objects, features, and advantages of embodiments of this invention will be apparent to persons of ordinary skill in the art from the following detailed description of the invention and the accompanying drawings.